1. Field of the Invention
The present invention is related to the field of marine towing operations. More specifically, the present invention is related to floatation devices, such as tailbuoys, used to maintain marine cables at desired depths and which have deployable masts to provide improved visual identification and on which transmission antennae are mounted for the communication of positioning information.
2. Description of the Related Art
In the field of marine seismic exploration, it is important to determine the configuration of the rock strata underlying the subsea earth's surface to locate subsurface structures favorable to the accumulation of oil and gas. In marine seismic surveying, this is accomplished by generating acoustic pulses or shock waves with sound sources, such as air guns, and by monitoring the resultant acoustic waves which reflect off the subsea interfaces with acoustic sensors. In a typical marine surveying operation, the seismic sound sources and the acoustic sensors are towed in designated patterns behind a seismic vessel. The basic principles of these surveying operations are well known to those skilled in the art.
As the vessel towing the acoustic sensors moves over the area being surveyed, the seismic sound sources are activated which introduces seismic signals into the body of water. The signals propagate downwards through the water and into the subterranean geological formations. Some of the signals are reflected by the interfaces between adjacent subterranean formations including the interface between the water and the floor of the body of water. The reflected signals travel upwardly through the geological formations and the body of water to the seismic receiver cable which is located near the surface of the body of water. The acoustic sensors or receivers detect the reflected acoustic waves and provide optical and/or electrical signals representative of such acoustic waves. The signals from the acoustic sensors are collected and relayed to apparatus aboard the vessel. These signals then are analyzed to provide information concerning the structure of the subterranean geological formations and possible oil and gas accumulations within the formations. The information typically is processed into display maps which show the thickness and orientation of the various strata underlying the sea bed.
Commonly, an array of seismic acoustic sensors, such as hydrophones, are configured in a seismic cable where the hydrophones are spaced along the length of the seismic cable. This seismic cable typically is called a streamer cable.
To optimally develop 3-D marine surveys, to reduce the number of passes required of the seismic vessel in surveying a specific area and to improve the quality of the resulting geophysical information, multiple streamer cables typically are deployed in a pattern parallel to the centerline of the vessel. The streamer cables are separated from each other by calculated offset distances to provide the desired, spaced parallel pathways which minimize duplicate coverage but are adequate to cover the area to be surveyed. To obtain and maintain the desired lateral distances between adjacent streamer cables throughout the time period during which the seismic vessel is traversing the survey area, the streamer cables are attached at predetermined tow points on the cable to devices referred in the art of seismic exploration as pullavanes or paravanes. The pullavanes are towed to the side of the vessel and provide the means to tow the streamer cables along pathways parallel to but laterally spaced from the pathway of the towing vessel.
Armored, optical-electrical towing cables, referred to in the art as leadin cables, are used to couple the streamer cables to the apparatus on the vessel which includes the power source and the data control system. In conventional towing arrangements, a tow line connects the leadin cable directly to the positioning device, such as a pullavane, which, as noted above, is positioned to the side of the vessel. The leadin cable forms a relatively large-radius loop behind the vessel (or from a boom or outrigger extended sideways from the vessel) to the tow line of the pullavane and then extends behind the pullavane to the point where the leadin cable connects to the streamer cable. Such an arrangement or configuration positions the streamer cable at the desired, controlled lateral offset from the vessel and from the adjacent streamer cables. Other streamer/leadin cable combinations are towed directly behind the vessel without the need for offset-positioning apparatus.
The streamer cable typically is filled with a fluid which acts as a buoyancy material to keep the streamer cable at the desired depth beneath the surface of the water during the surveying operation. Because of the length of the streamer cables (sometimes several miles in length), the streamer cables are in danger of being snagged by other vessels. Therefore, a floatation device, such as a tailbuoy, is attached to the submerged, tail end of the streamer cable to provide means to visibly approximate the location of the end of the streamer cable. The tailbuoy is also quite useful for retrieval operations. If the vessel-end of the marine cable becomes detached from the vessel, the marine cable can be retrieved from the tailbuoy-end of the cable by using the tow line attached to the tailbuoy and the streamer cable.
Additionally, the tailbuoy commonly contains equipment for receiving data from a positioning system, such as the satellite navigation system known as the Global Positioning System (GPS), processing the data and transmitting the tailbuoy's position information to a tracking system on the vessel. Antennae, for receiving the signals from the satellites and for transmitting the processed signals to the vessel, are mounted on masts which extend upward from the tailbuoy to provide as much height as possible to minimize the negative effects of waves and other surface conditions on the signal transmission between the communication equipment on the tailbuoy and the control equipment on the vessel. The tailbuoy's positioning data not only provides a means to physically locate the tailbuoy but can also be used to assist in determining the actual position of the end of the streamer cable. Increased accuracy in the calculated position of end of the streamer cable, and thereby increased accuracy for the positions of the acoustic sensors in the streamer cable, provides for increased precision in correlating the seismic signals received by the acoustic sensors to actual earth formations.
A typical tailbuoy used for relaying position information to the vessel, and without equipment for steering, may be approximately four feet by six feet in size and weigh over 2000 pounds. Because of the bulkiness of such an apparatus, any possible trimming operations, such as positioning the mast into its horizontal storage position and removing the antennae, are done prior to storing the tailbuoy on the vessel. The antennae are reattached and the masts are deployed to their vertical positions for operation as the tailbuoy is hoisted from the deck of the vessel and deployed over the side or rear of the vessel into the water.
One of the problems with the prior art tailbuoys is that the masts on these tailbuoys are deployed to their desired vertical positions after the tailbuoys are deployed from the deck of the vessel. The prior art method involves positioning the tailbuoy over the side or rear of the vessel and then having crew members lean over the side or rear of the vessel to ratchet the masts into their desired vertical, deployed positions. The inherent problems caused by rough seas and crew members unable to maintain their balance while performing this maneuver and the amount of time required for mast operations (ratcheting each mast on each tailbuoy to its desired vertical position prior to starting the surveying operation and then ratcheting each mast back to its storage position after the surveying operation) have focused attention on the need for a safer and quicker method of deploying and deactivating the masts.